Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
1242 PM CDT Monday Aug 3 2015
Short term...(today and tonight)
issued at 349 am CDT Monday Aug 3 2015
Today and tonight...surface analysis depicts a cold front from the eastern Great
Lakes west-southwestward over southern Michigan to northern Illinois and southern Iowa. It is this front
which pushed through late yesterday resulting in the much cooler/drier air
experienced this morning with clear skies. This cooler/drier high pressure
airmass will remain over the area through tonight...keeping much of the
cloud cover associated with the deep Hudson Bay upper level low to the
east of the weather forecast office mpx County warning forecast area. That said...some wobbling of the large upper
low with several shortwave spokes of energy rotating around the upper
low may bring some cloud cover to far northestern portions of the County warning forecast area
this afternoon but rain showers are expected to remain well off to the
east. The upper low will begin to move further east tonight while ridging out
west expands slightly eastward...making for clear skies upcoming tonight
into Tuesday morning. As alluded to at the start...temperatures will run
noticeably cooler and humidity levels will be much lower. Highs
today will hit the middle-upper 70s while dewpoints cap out in the lower
50s...and this will be followed by lows early Tuesday morning in the upper
40s to middle 50s.
Long term...(tuesday through sunday)
issued at 349 am CDT Monday Aug 3 2015
The long term period starts off quiet and pleasant. With the upper
low still parked over Hudson Bay...the upper Midwest will continue
to be fed dry northwesterly flow from Canada as surface high
pressure stays with US through the day. High temperatures
approaching 80 with dew points in the low 50s will feel very
comfortable for most. There could be some cumulus development
during the day as cumulus rule indicates slightly negative values in
eastern Minnesota and western WI...coinciding with cyclonic circulation
on the back side of the long wave trough.
The forecast becomes quite murky Wednesday and beyond.
Disagreement among the latest deterministic solutions was
pronounced. While the GFS would indicate Theta-E advection and pv
advection causing showers and thunderstorms Wednesday for western
Minnesota...the ec holds that activity off completely...and the NAM is a
tad slower than the GFS and keeps most of the precipitation west of our
area. The GFS has become an outlier in how quickly the wave moves
east...so trended the forecast toward the ECMWF/NAM...meaning
bringing the precipitation in slower...but with all the spread in the
solutions...kept some 20 to 30 percent probability of precipitation to extreme western Minnesota
during the day on Wednesday...mainly late afternoon. On Wednesday
night...the ec and Canadian indicate decent agreement in working a
middle level wave from ND across Minnesota by Thursday morning...and
spreading precipitation through during that time...but the NAM is
completely dry and the GFS wants the precipitation confined to SW Minnesota with
the vorticity maximum from the potential precipitation earlier in the day.
The ec and GFS finally do agree on another shortwave quickly
moving in Thursday afternoon and Thursday night spreading broad
lift across the area and a shot for additional rainfall.
In reality...due to the differences in the guidance...there is a
lot of 20-40 percent probability of precipitation currently in the forecast from Wednesday
through Friday...but once the guidance can start to offer a better
sense of what we can expect to actually occur...probability of precipitation will likely
be able to be consolidated spatially and temporally.
Confidence in the weekend precipitation forecast is extremely low
given the variability in the guidance and how the various
solutions for handling the waves we first must deal with
Wednesday through Friday. The GFS and ec both indicate a closed
upper low progressing east-southeast from southern Canada through
northern Minnesota...but differ in both placement and timing. The generic
allblend of the models now smothers the entire area in 20-30 probability of precipitation
for the entire weekend...largely due to the timing differences in
the handling of the upper low. We'll have to carry these low
end/low confidence probability of precipitation until more clarity can be gained from the
guidance in future runs.
Aviation...(for the 18z tafs through 18z Tuesday afternoon)
issued at 1242 PM CDT Monday Aug 3 2015
VFR through the period. Few-sct050-070 this afternoon along with northwest
winds 12-15 knots gusting 18-24 knots. Highest wind speeds in WI.
The winds will diminish quickly this evening.
Kmsp...VFR through the period.
/Outlook for kmsp/
Wednesday...mainly VFR. Chance MVFR shra/tsra. Wind east 5-10 kts.
Thursday...mainly VFR. Chance MVFR shra/tsra. Wind southeast 5-10 kts.
Friday...mainly VFR. Chance MVFR shra/tsra. Wind east 5-10 kts.